1. Field of the Invention
The present invention relates to a method of storing optically generated charges by an optical signal in a solid state imaging device using a MOS image sensor of a threshold voltage modulation type, which is used for a video camera, an electronic camera, an image input camera, a scanner, a facsimile or the like.
2. Description of the Prior Art
Since a semiconductor image sensor such as a CCD image sensor and a MOS image sensor is excellent in mass productivity, the semiconductor image sensor has been applied to almost all types of image input devices accompanied with the progress in a fine pattern technology.
Particularly, in recent years, the applicability of the MOS image sensor has been recognized again because of the advantages thereof, i.e., smaller power consumption compared with that of the CCD image sensor, and the capability of making a sensor element and a peripheral circuit element by the same CMOS technology.
In consideration of such a social trend, the present inventor has improved the MOS image sensor, and filed a patent application (Japanese Paten Application No. Hei 10-186453) regarding a sensor element having a carrier pocket (high-density buried layer) 25 under a channel region of an insulated gate field effect transistor for optical signal detection (hereinafter, referred to as a MOSFET for optical signal detection or simply as a MOSFET in some cases) to obtain a patent thereof (Registered Number 2935492).
The above-described MOS image sensor has a circuit constitution shown in FIG.8(a) of the patent (Registered Number 2935492). A series of operations of the MOS image sensor are made to pass through a storing period, a reading-out period and an initializing period as shown in FIG. 8(b). During the storing period, optically generated holes are generated by light irradiation and then stored in a hole pocket 25. During the reading-out period, an optical signal proportional to the storage amount of the optically generated holes is detected. Then, during the initializing period, a high reverse voltage is applied to each electrode, i.e. gate, source and drain, and optically generated holes stored in the hole pocket 25 are discharged to perfectly deplete a hole storage region.
The present inventor has filed various novel applications related to the invention according to the patent (Registered Number 2935492). According to these applications, as shown in FIG. 8 of this application, during the storing period, a potential of a gate electrode (Vpg (VSCAN)) is set to be a low voltage, that is, a ground potential in general, and a drain potential (Vpd) and a source potential (Vps) are set to be higher than the gate potential, that is, about 3.3 V in many cases. A depletion n-channel MOSFET for optical signal detection is made to maintain a cutoff state (depletion state) in the above-described manner, and then optically generated charges generated in a photo diode are transferred to the carrier pocket 25 under the channel region.
However, during the foregoing storing period, carriers of the same type as the optically generated charges may be sometimes discharged from levels on an interface between a gate insulating film and the channel region. In this case, the carriers discharged from the interface levels flow into the carrier pocket 25 to be a leak current. Therefore, holes other than the optically generated charges are stored in the carrier pocket 25.
In such a case, if making patterns finer for increasing a photosensitivity, a large amount of holes are stored in the carrier pocket 25 even by a little leak current that has not conventionally brought a problem, thus there is a fear of causing a problem that a bright luminance line, that is, a so-called white scratch, is generated on an image screen.
Alternatively, even if only the optically generated charges, which are not generated by a leak current, are normally stored in the carrier pocket 25, there may be some cases where excessive optically generated charges than necessary are stored therein.
In this case, a non-selected unit pixel (cell) (Vpg=0) comes into a deep depletion state by the excessively stored optically generated charges, thus raising a source potential. On the other hand, in a selected cell, in the case where the stored charges are at a low level, the source potential is lowered, and a margin of the selection and discrimination is reduced, thus causing worsening of a so-called smear characteristic, that is, generation of a bright band with striped-shape in a perpendicular direction on the image screen.